In connection with offshore oil and gas exploration where floating vessels such as drilling rigs or drilling ships are connected to the seabed via a drill string, riser or any other seabed connected equipment, it is a need for compensation systems suitable for compensating movements of the floating vessel normally caused by natural phenomena such as waves and winds. If there were no such compensating systems the seabed connected equipments could be damaged or deteriorated due to for example buckling and/or tension.
A central part of this compensation system is one or more hydraulic pressure cylinders which is in fluid connection with one or more piston accumulators. The purpose of the piston accumulators is to ensure that the high pressure chamber within the hydraulic pressure cylinder is maintained under pressure. The piston accumulators are normally of air/oil type and comprise a floating accumulator piston separating hydraulic fluid in one part of the accumulator from the compressed air (or other fluids) in the other part of the piston accumulator. The latter part, e.g. with the compressed air, may be connected through a pneumatic pressure line to a high pressure gas bank of compressed gas used to maintain a predetermined high pressure level in the piston accumulator. Further, the hydraulic fluid is in fluid communication via a hydraulic pressure line to the high pressure chamber of the hydraulic pressure cylinder. When the compensating system is being operated, hydraulic fluid is forced from the piston accumulator through the pressure line and into the high pressure chamber of the hydraulic pressure cylinder where a hydraulic force is exerted on the face of a cylinder piston. The pressure developed on the floating accumulator piston maintains the hydraulic fluid pressure in the piston accumulator and the high pressure chamber of the hydraulic pressure cylinder as the volume of the high pressure chamber changes. Hence, when the floating vessel experiences movements, such as heaves downward relative to the riser, the cylinder housing is displaced down with respect to the cylinder piston, thereby increasing the volume of the high pressure chamber of the hydraulic pressure cylinder.
For the detection and recording of the position of the floating piston in the piston accumulators, it is generally known to use distance measuring devices such as an antenna which can be used to measure the distance between the piston(s), serving as a reflecting body inside the piston accumulator, and the accumulator base which holds an antenna. The detection of the piston position in accumulators can be performed either discrete, i.e. at discrete sites, or continuous, i.e. continuously during operation. The antenna and antenna cable are normally formed as a part of the end cover of the accumulator or is mounted between the high pressure chamber in the accumulator and the end cover. Prior art solutions include arrangements wherein the whole upper end, including e.g. flanges and end covers, of the piston accumulator has to be removed if a minor part such as an antenna or antenna cable must be replaced. The size and weights of these components, e.g. flanges and end covers, mounted on pressure cylinders are massive. Hence, to change or repair an antenna, transmittal line or other part, may prove difficult due to the dimensions and weights of the flanges and end covers.
It is therefore an object of the present invention to provide a solution that makes it easy to substitute or repair the antenna or transmittal line without the need of dismounting the larger elements making up the upper end of a piston accumulator.
Another object of the invention is to provide a solution where the antenna is kept centrally within the piston accumulator while removing the antenna and/or transmittal line.